Magnetic recording on bit patterned media (BPM) is seen as a way to increase the areal recording density compared to conventional magnetic recording while avoiding the problem of the super-paramagnetic limit. The surface of the recording medium includes a regular array of islands and trenches. Different geometries are possible for the bit island location including the staggered geometry.
In bit patterned media, the magnetic material on the disc is patterned into small isolated blocks or islands such that there is a single magnetic domain in each island or “bit”. The single magnetic domains can be a single grain or a plurality of strongly coupled grains that switch magnetic states in concert as a single magnetic volume. This is in contrast to continuous media wherein a single “bit” may have multiple magnetic domains separated by domain walls. The media may be fabricated so that there is no magnetic material in the regions between the blocks.
In magnetic storage media, bits are written on the magnetic medium by applying a magnetic field on the recording medium using the write head. The write head is controlled by a clock, which determines where the bit transitions occur. To read these bits (at a later time), a read head senses the magnetic field on the medium and converts it into an electrical signal, which is further processed. The read process also uses a clock, which can be the same clock as the write clock. However the two clocks are to be considered non-synchronous because they operate at different times. The read channel samples the readback waveform, and processes the samples to recover the bits.
Timing recovery is a mechanism used to determine where to sample the signal. The nominal sampling frequency is known, but because no two clocks are synchronous, it is desirable to determine exactly where to sample the signal using other means.
In magnetic recording systems that use continuous media, there is no need to synchronize the writing of data with the position of the media. For this reason, the data is written using a free running clock and sampling instances are recovered during the read process. However, for patterned media, the locations of the bits to be written are predefined (i.e., on the bit islands), and write synchronization is needed. Thus bit patterned media restricts the locations of the bits to be written. If the write head is not synchronized to the bit island location, there will be cycle slips. Even a single cycle slip would cause system failure. Read-while-write has been proposed for write synchronization, but the drawbacks are reader saturation, crosstalk, etc.
A challenge in the implementation of bit patterned recording is timing recovery or synchronization. The write head needs to be aware of positions of the bit islands in order to precisely switch the write fields as the head passes over a track. Similarly, the read head needs to recover the correct sampling instants from the readback waveform. Failure in write synchronization causes insertion/deletion errors of bits as well as bit flips in its vicinity.